1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine or printer using electrophotography or electrostatic recording. In particular, it relates to an image forming apparatus having temperature sensing means for sensing the temperature of a recording material after fusing.
2. Description of the Related Art
An image forming apparatus, such as a copying machine or printer, using electrophotography or electrostatic recording is provided with a fuser for heat-fixing or fusing a toner image onto a recording material. In this case, various techniques are employed to increase the fixability of the image.
One of such techniques, there is proposed a technique for sensing the temperature of a recording material after fusing, and feeding it back so that it will be brought close to a control target temperature. For example, see Japanese patent application laid-open No. H01-150185 (1989), Japanese patent utility model laid-open No. H01-160473 (1989), and Japanese patent application laid-open Nos. H03-53276 (1991), H04-181250 (1992), H06-308854 (1994), H07-230231 (1995), H07-239647 (1995), H10-161468 (1998), 2000-66461, 2001-13816, 2002-23555, 2002-214961, and 2003-29485.
For example, FIG. 9 shows an example of a fusing device using a non-contact sensor for sensing the temperature of a recording material after fusing. In such a fusing device, a non-contact sensor 20 such as an infrared sensor is placed downstream of a fixing area or nip so that it measures the temperature of the recording material in a non-contact manner.
FIG. 10 shows an example of a fusing device using a contact type sensor for sensing the temperature of a recording material after fusing. In such a fusing device, a temperature sensor 18 such as a thermistor is placed downstream of a fixing nip, and an opposed member 19 such as a rubber roller is placed to face the temperature sensor, so that the recording material is nipped between the temperature sensor and the opposed member, allowing the temperature sensor to measure the temperature of the recording material.
In the structure for sensing the temperature of the recording material to feed back the sensed temperature, temperature sensing accuracy is crucial.
Upon fusing the recording material, since the moisture contained in the recording material is also heated, water vapor is produced from the surface of the recording material. In the case of temperature detection using the non-contact sensor, the water vapor fogs the surface of the non-contact sensor, and this makes it difficult to detect the temperature of the recording material accurately.
On the other hand, in the method of bringing the opposed member into contact with the temperature sensor so that the temperature sensor can detect the temperature of the recording material nipped between the temperature sensor and the opposed member, the opposed member draws heat from the recording material, and this also make it difficult to detect the temperature of the recording material accurately.